The present invention relates to heart pacemakers, and in particular to a circuit for testing the remaining battery life of the battery in a heart pacemaker.
In the use of implantable heart pacemakers, it is important to be able to identify the remaining useful life of the battery, and thus of the heart pacemaker, in a simple manner. It is known to provide a heart pacemaker with circuitry which measures the internal resistance of the battery, thus giving an indication as to the remaining life. For this purpose, the pacemaker is switched to a "test mode." The status of the battery is indicated by generating a series of test pulses at a frequency corresponding to the battery resistance, the frequency being measured externally.
A battery testing unit for a heart pacemaker having a LiI cell is known, wherein the battery is loaded with a known value and the resulting voltage drop is measured. Depending upon the voltage drop, and thus upon the internal resistance of the battery, the stimulation frequency is changed during the test mode, and the stimulation frequency is then externally measured. A test unit of this type, however, is suitable only for a battery having an internal resistance which increases as the battery discharges.
It is an object of the present invention to provide a test circuit for the charge status of the battery of a heart pacemaker which is suited for batteries having a constant internal resistance during at least a portion of their useful life.
The above object is achieved in accordance with the principles of the present invention by a circuit including a stimulation pulse counter and input logic which identify the consumed charge from the pacemaker operating values, from the number of stimulation pulses emitted, and from a chronological base value. The pulse counter and input logic are connected to a memory or charge counter in which the charge which has been consumed since a preceding test is summed. Identification of the charge status of the battery thus does not occur by means of a load connected to the battery, but instead by measuring the charge drawn from the battery, added to a base value, which is summed in a memory. The content of the memory thus provides information regarding the remaining useful life of the battery. The circuit is suitable for all types of batteries, and specifically for batteries having an internal resistance which remains constant over a relatively long period despite discharging of the battery.
The circuit can be constructed as an implantable unit and internally executes all calculations and need only have means for externally reporting a final value which corresponds to the charge status of the battery. It is also possible, however, to undertake the necessary calculations in an external programming device which communicates with an implanted unit which simply acquires the values necessary to make the calculations. The programming device updates the values in ause register in the implantable unit.
Display of the charge status of the battery can be undertaken such that an electrical signal corresponding to the charge status controls a pulse generator which generates at least one marking pulse at the output of the heart pacemaker, the marking pulse having a chronological position with respect to the stimulation pulses which is dependent upon the charge status of the battery. Measurement of the charge status and thus of the remaining useful life of the battery is thus possible by observing the output pulses of the heart pacemaker, and has the additional advantage that the frequency of the stimulation pulses need not be altered during a test mode.